Arcing in electrical systems is a well-known but unwanted phenomenon, typically resulting from either poor installation procedures for system components, or breakdown of conductors or insulators in the system, creating an arc pathway between two conductors or to ground. Arcs may damage electrical system components immediately or over time and may cause potentially detrimental circuit breaks. If an arc has sufficient current and voltage, it may become a sustained arc that is either substantially constant or recurs at regular or irregular intervals. A sustained arc is desirable for applications such as welding, but in other applications an unwanted sustained arc can melt, corrode, or otherwise damage system components, and can reduce the overall performance of the system. In some electrical systems, a sustained arc may be produced at very small current levels.
Self-regulating electrical systems can be susceptible to this phenomenon. Such electrical systems can contain polymer-based, semi-conducting components which derive their conductivity from the addition of carbon black, carbon nanotubes or other conductive materials. In particular, self-regulating heating cables used in trace heating applications can be susceptible to this phenomenon when improperly installed or damaged. In such a cable, parallel current-carrying bus wires are normally spaced apart from each other. In solid core heating cables, the bus wires are separated by a semi-conductive, substantially solid polymer heating element. In fiber heating cables, the bus wires are insulated from each other by a non-conductive spacer, and a semi-conductive fiber heating element is wrapped helically around the bus wires. The heating cable does not exhibit arc faults under normal conditions. However, if the cable is installed improperly or damaged, the bus wires may become partially exposed and a sustained arc may be created between those exposed wires if they are exposed to water and energized. For example, if the cable is installed in a wet area, the exposed bus wires may contact or be submerged in water. Depending on the specific circumstances, the water can initiate a sustained arc that is not able to trip a circuit protection device, but nevertheless may destroy the heating element and release conductive particles. These particles can exacerbate the arcing and do other damage.
Electrical systems often employ circuit protection devices, including arc detectors that place the system in a “fault” condition, such as by tripping a circuit breaker or initiating an alarm when the detector detects an arc that exceeds a certain threshold or contacts a certain component. Conventional circuit breakers are designed to protect electrical circuits by detecting overloads and short circuits. The amount of current transferred in these situations is high, so these devices have a low sensitivity to current variations and thereby avoid false alarms that would break the circuit without need. In contrast, a residual-current device (“RCD”) is configured to disconnect an electrical circuit when the device detects an excessive imbalance in system current that can be caused by arcing transfer of current to a conductor that normally does not carry system current. RCDs, including ground fault interrupters (“GFIs”), earth leakage circuit breakers “ELCBs”, safety switches, and trip switches, are configured with a much lower sensitivity than a conventional circuit breaker and are able to detect arc-induced erratic circuit behavior that does not trip a breaker. At such a low sensitivity, the RCD must be further configured to differentiate between an arc-induced current variation and one caused by normal circuit operation, such as the actuation of a switch, the activation or deactivation of a motor, or the sudden removal of a load by unplugging or other means, in the electrical system. This adds cost and complexity to RCDs, and false positive circuit interruption remains a major drawback for existing RCDs. Yet a third type of device, an arc fault circuit interrupter (“AFCI”), may detect variations in the current in both frequency and time which are not characteristic of any regular electrical loads. However, while they can be more sensitive than some RCDs and can also detect arcs between load and neutral terminals not involving ground, experimental tests have shown that AFCIs are still not sufficiently sensitive to detect a sustained arc in commercial self-regulating heating cables.
In particular, attempts to deploy known commercial AFCIs to detect sustained bus-to-bus arc faults as well as ground faults and other arc faults in electric heat tracing systems, including self-regulating polymer-based heating cables, have failed.